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1.
When two or more males gather around a single toilet or urinal to see how many times each flow of urine overlaps until all participating males' bladders empty.
"Hey, Chad. Want to go in the bathroom and Cross the Streams? Bill and I counted to 59 last time. A new record!"
by GiraffePoacher August 15, 2011
 
2.
Crossing the streams: When you're urinating and defecating at the same time, and you maneuver your body just right so you can urinate on your feces while it's coming out.
This isn't a sexual act, but I'm sure someone can figure out a way.
"Dude, I just pissed on my shit while it was still coming out. It was awesome!"
"Yeah man, I cross the streams all the time."
by GhostGuyGGG July 19, 2009